AND CLASSIFICATION OF THE HETEROPODA. 15 



represented as bifid, having an anterior part, conical and grooved, and a posterior 

 apparently glandular and trumpet-like segment (Plate II. fig. 1, m). I have 

 never been able to trace the vas deferens to this organ in any of the Heteropods ; 

 and having found the mantle-cavity full of spermatozoa in Carinaria, I am dis- 

 posed to think that the scheme must be similar to that which obtains amongst 

 ordinary gasteropods, when the penis is imperforate. 



The ovary in the females is very similar to the testicle, both in shape and 

 position, but the contained ova at once decide the difference, and the large thick- 

 walled and convoluted oviduct, without the dark-coloured fusiform dilatation 

 characteristic of the male sperm-duct, still further settles the question of sex. 

 The aperture of the oviduct would appear to lie deep within the mantle. I could 

 discover no opening in the position occupied by the penis in the male, though I 

 have observed ova escape from beneath the margin of the mantle.* 



The characters of the operculum are also significant, — viz., as to its general 

 form, the course of the lines of growth, the position, size, and appearance of the 

 nucleus, the muscular impressions, and its smooth or dotted surface. 



Oxy gyrus (Benson). 



The genus Qccy gyrus, established by Mr Benson, includes the Atlantce of 

 Rang, Kerandrien, and Lamanon, characterised by having an involute instead of 

 a spiral nucleus to the shell, greater fulness in the whorls, and a comparatively 

 small amount of calcareous matter as a component, particularly near the mouth 

 and in the keel (Plate II. fig. 3). 



The most striking difference that I have noticed between the soft anatomy of 

 Atlanta and of Oxy gyrus is that the testis is short and broad, lying at the base of 

 the liver in the latter case ; whereas it is much elongated in the former, extend- 

 ing far inwards between the liver and the columnellar wall of the shell. 



The rachidian plates in all the Oxygyri, instead of a single dental process of 

 Atlanta,\)Q3iY three conical teeth on their posterior border; and these teeth are either 

 all large and nearly equal in appearance (Plate II. fig. 5, e), or the middle tooth 

 alone is well developed, while the lateral ones are rudimentary (Plate II. fig. 5, f). 



The pleurse are essentially similar to those of Carinaria and Cardiapoda, pre- 

 sently to be described, the most important character in all being the presence 

 of a small conical, and often incurved tooth (Plate II. fig. 7, i'), on a little shoulder 



* All the species of Atlanta present so close a general resemblance, that then* nice discrimina- 

 tion requires cai-eful examination and comparison. The keel may be more or less, or not at all inter- 

 posed between the peristome and the body whorl. It may be plain or wavy, though the shell never 

 exhibits this latter character, as it does in Carinaria. The spire, as to its prominence, depression, 

 evenness or obliquity, closeness or openness of its whoi-ls, smoothness, dotted surface, or linear mark- 

 ings, affords us characters which do not appear to have been hitherto sufficiently recognised. There 

 is, moreover, in colour, which one would naturally regard as being of little importance, a scarcely 

 ever failing peculiarity of species. 



